1. Field of the Invention
This invention relates to a magnetic disk for magnetically recording signals on its surface in a direction parallel with the surface or in a direction perpendicular to the surface.
2. Background Art
One example of magnetic disk of this type is a rigid magnetic disk. The substrate of the rigid magnetic disk is made of a rigid material. Usually, an aluminum plate (JIS A5086 for instance) is employed as the substrate of the rigid magnetic disk.
The rigid magnetic disk is generally manufactured as follows. The aluminum plate is machined on a lathe to form a disk. In order to minimize the space between the magnetic head and the disk to thereby allow high density recording, the surface of the disk is polished. A magnetic layer is then formed on the thus polished surface by vacuum deposition or spin coating. In this case, the surface of the disk should be as smooth as possible for a high density recording and reproducing operation. However, it has been difficult for the conventional aluminum disk to have a surface which is not more than 0.1 micrometer in central line average roughness Ra. Furthermore, it is considerably difficult to handle the aluminum disk on the manufacturing line as it is not flexible. For instance, in forming a magnetic layer, it is impossible to continuously coat it with magnetic material. In addition, as the high density recording operation is greatly affected by dust on the surface of the disk, adherence of dust onto the disk must be prevented during the manufacture. Thus, the manufacture of rigid magnetic disks is intricate and considerably difficult and requires a great investment in equipment.
The conventional substrate is rigid since it is made, for instance, of aluminum as was described before. On the other hand, the magnetic head should trace the magnetic layer of the disk with a narrow space therebetween. It is rather difficult to maintain the narrow space unchanged, and therefore signal errors often take place. It is essential to reduce the space between the magnetic head and the disk surface. However, it is considerably difficult to do so. As the space is narrow as was described above, the magnetic head may contact the disk surface. If this malfunction occurs, since the substrate is rigid, a great impact is applied collectively to the part of the magnetic layer which the magnetic head has contacted. As a result, the magnetic layer may be broken. That is, the service life of the disk may be reduced.
Furthermore, the aluminum substrate polished as described above is expensive.
Recently, a magnetic disk has been proposed in the art which is manufactured as follows. Annular wide recesses are formed in both sides of a disk substrate. Two floppy disks or flexible disks (hereinafter referred to as "flexible disk sheets"), each of which has a magnetic layer on its one side, are bonded to both sides of the disk substrate in such a manner that the magnetic layers are exposed to the outside and gaps are formed between the disk substrate and the flexible disk sheets.
The magnetic recording surfaces of the rigid magnetic disk thus manufactured, unlike those of the rigid magnetic disk, are flexible. Therefore, when the magnetic head accidentally touches the magnetic recording surface or when a high density recording operation is carried out with the magnetic head in contact with the magnetic recording surface, the magnetic layer is scarcely broken as opposed to the case of the rigid magnetic disk. Accordingly, the flexible disk technique can be utilized as it is, and a smooth durable magnetic layer can be employed. Thus, the magnetic disk of this type is being watched with keen interest.
The inventors have conducted intensive research on the magnetic disk described above and have found that it still suffers from serious difficulties. Namely, if, when the flexible disk sheets are bonded on the substrate with adhesive, the quantity of adhesive is too small, then the flexible disk sheets may come off the substrate. On the other hand, if the quantity of adhesive is too large, the flexible disk sheets are liable to be creased or the adhesive is forced out of the magnetic disk.